1. Field of the Invention
This invention relates to a device which regulates the gas pressure within the annular collapsible and inflatable cuff of a cuffed endotracheal tube, which is a therapeutic apparatus for artificial ventilation and anesthesia. More particularly, the invention relates to a pressure regulator for a cuff of a cuffed endotracheal tube to maintain the intra-cuff gas pressure at a certain, but adjustable, safe and effective level despite variations in breathing pressure and/or variation of the ambient gas pressure. This device permits to automatically superimposing each breath-synchronized pressure variation on the basic intra-cuff pressure of the cuff being inflated along the endotracheal tube in the trachea so as to be set at an appropriate and constant pressure above the ambient pressure.
2. Description of the Prior Art
When a tube is inserted into the trachea of a patient for artificial ventilation or for inhalation of anesthetic gas (such a tube will be referred to as an "endotracheal tube", hereinafter), a ring-shaped hollow collapsible soft bag is usually attached as a cuff to the patient end of the endotracheal tube and the cuff is inflated by blowing a gas therein so as to hold the endotracheal tube in position while keeping airtight contact between the inside surface of the trachea and the endotracheal tube.
Previously, the so-called high-pressure cuff was used; namely the cuff is inflated by a comparatively high pressure gas. The high-pressure cuff has a shortcoming in that it may press against the internal wall of the trachea with a pressure so strong that it causes a disturbance in the trachea. Accordingly, low-pressure cuffs with less risk of causing a disturbance, which are inflatable by a large amount of low-pressure gas, have been used increasingly.
However, the low-pressure cuff has a shortcoming in that, due to its large surface area and the thin membrane forming the soft bag, the gas being inhaled through the endotracheal tube, such as an anesthetic laughing gas for general anesthesia, tends to diffuse into the cuff, resulting in an increased pressure within the cuff.
To overcome this shortcoming relating to the intra-cuffpressure (sometimes, referred to as "cuff pressure", hereinafter) of the conventional cuffed endotracheal tube, various cuff pressure regulators have been proposed, for instance by U.S. patent application Ser. No. 879,437 of the inventors, and others. Such cuff pressure regulators intend to maintain a constant cuff pressure regardless of occurrence of factors affecting the cuff pressure; such as variation of the tonicity (cough, excitement, etc.) and faccidity (relaxation) of the patient, variation of ambient pressure and ambient temperature.
When the cuff pressure is kept low with or without the cuff pressure regulator, if the positive pressure within the respiratory organs of the trachea increases intermittently, for instance as the result of artificial respiration, there is a risk that gases used for anesthesia and for artificial respiration can leak over the cuff because the pressure urging the cuff to the internal wall of the trachea is low. For similar reasons, there is a risk that when the pressure of the gas ventilated into the respiratory organs is high, such high pressure may act on the deeper side (towards the lungs) surface of the cuff in such a manner that the endotracheal tube being used may be pushed, transformed deviated and finally dropped out from the trachea.
To minimize such risks resulting from use of the low-pressure cuff, it is necessary to fasten the endotracheal tube by a suitable means or to increase the cuff pressure sufficiently to prevent the inadvertent drop out of the endotracheal tube and gas leakage. Consequently, the low-pressure cuff is not free from the risk of causing a disturbance on the internal wall of the trachea.
To prevent the above-mentioned gas leakage during artificial respiration and the inadvertent drop out of the endotracheal tube due to the pressure of the ventilated gas, methods for varying the cuff pressure with variation of the ventilatory pressure have been proposed. In the conventionally proposed methods, only when the pressure of the gas being ventilated into the respiratory organs is high, is a certain gas pressure added to an arbitrarily set basic cuff pressure. More particularly, the pressure of the respiratory organ is selectively added to the inhaling gas pressure toward the cuff, corresponding to the arbitrary set basic cuff pressure, through a diaphragm or a piston. However, the diaphragm is not durable because it is a thin membrane, while the piston involves problems such as resistance against movement and gas leakage. Further, even if the basic cuff pressure is set at a proper level beforehand, it cannot be used in a hyperbaric therapeutic chamber because the sudden ambient pressure change necessitates modification of the basic cuff pressure.
With respect to anesthesia using an endotracheal tube, there is a method in which gas with a pressure corresponding to the intermittent positive endotracheal pressure, caused by an artificial respirator or the like, is fed into the cuff of a cuffed endotracheal tube, so as to prevent leakage of the respiratory gas. When this method of preventing gas leakage is actually used, during the intermediate period between the intermittent gas supplied by the artificial respirator (lung ventilator) or the like, the endotracheal pressure is reduced to a level substantially equivalent to the ambient pressure, and the cuff pressure is similarly reduced to the same level as the ambient pressure, and the adhesion of the cuff to the internal surface of the trachea becomes insufficient. Thus, there is a serious risk in that, if phlegm, sputum, or vomitus is somehow brought to the trachea, they may be aspirated.